A permanent magnet brushless motor in which a concentrated winding wire is made on every tooth thereof is widely used in an electrical home apparatus, audio equipment, an information apparatus, transportation equipment and the like. With this type of motor, torque ripple or cogging torque occurs with magnetic attraction exerting over permanent magnet and teeth, causing a vibration and a noise problem.
To obtain a low vibration and a low noise motor, methods of reducing the cogging torque have been practiced, such as optimization of number of poles and teeth, skewing, and magnetization of the magnet in sinusoidal waveform. For examples, patent document 1 discloses a method of reducing cogging torque by determining a relation between number of pole P and number of teeth M to P:M=10:12. Patent document 2 discloses a method for reducing torque ripple by determining a relation between number of pole P and number of teeth Q to P/Q>0.8, and a relation between teeth tip width bt1 and pole pitch τρ to bt1/τρ≤0.8. Patent document 3 discloses a method of reducing the cogging torque, with a motor having a P to Q ratio of 3:4, by determining a teeth width at an electrical angle of 145 to 165 degrees and 85 to 105 degrees, without sacrificing an output torque. Patent document 4 discloses a structure in which a shape of a magnet arranged on a surface of rotor is made thicker toward a center of the magnetic pole but gradually made thinner toward a space between magnetic poles. By taking the structure, patent document 4 aims a pattern of magnetic flux density on the surface of the rotor to come closer to a sinusoidal waveform, thereby reducing cogging torque and achieving a low vibration and a low noise. Patent document 5 describes an idea of magnetizing the magnet in essentially a sinusoidal waveform instead of adjusting configuration of the magnet, aiming a surface magnetic flux waveform forms a lean sinusoidal waveform and a surface magnetic flux in a neighborhood of pole boundaries becomes essentially zero. By magnetizing the magnet in this pattern, patent document 5 aims to decrease cogging torque and therewith realize a low vibration and a low noise.
However, with above mentioned conventional technologies, although cogging torque or torque ripple causing noise and vibration is somewhat reduced, motor efficiency is not yet specifically addressed. The conventional technologies are therefore not complete enough as a motor for home apparatus such as a fan motor of air conditioner where demand not just for a low vibration and a low noise but for a high efficiency is increasing year by year, leaving a task. Further, adjusting the shape of the magnet as in patent document 4 is difficult in production. Furthermore, for securing a large magnetic flux density, a thick magnet is required, increasing a usage amount of the magnet therefore an increase in cost, leaving a task. Still further, with the magnetizing method described in patent document 5, a neighborhood of pole boundaries is not fully magnetized, so magnetic power of the magnet is not effectively utilized, lowering the efficiency of the motor, leaving another task.
With a conventional fan motor for air conditioner, a link type divided core is traditionally used in which divided cores are linked together to enhance a motor efficiency and to increase production efficiency. This structure allows a winding nozzle to fully utilize a nozzle passing space, making an aligned winding of wire possible and therefore a high density winding possible. The increased winding enhances torque, decreasing cooper loss and so increasing efficiency of the motor. However, with such a configuration, a minute gap may be created when divided cores are linked together, the gap space decreasing torque, leaving a task. Still further, when linking the divided cores, a dimensional difference is likely to appear in an inside periphery of the divided core because of a work precision of the divided core and assembling error of the core, creating an uneven gap distribution between the stator and the rotor, making magnetic variation large, hence causing the vibration and the noise large, leaving still other task.